1. Field of the Invention
This invention relates to a process of thermally treating bulk material with hot gases in a rotary kiln below the melting point of the charge components in the rotary kiln while cooling the shell and refractory lining of the kiln.
2. Discussion of Prior Art
In such processes carried out in a rotary kiln, for instance during a direct reduction of iron ore to produce sponge iron or during the volatilization of non-ferrous metals from metallurgical plant residue with simultaneous reduction of the iron oxides to produce sponge iron in a direct reducing process or waelz process or during roasting or calcining processes or the production of quicklime in the rotary kiln, heat is transferred from the hot gas to the solid charge
(a) by radiation from the gas, PA1 (b) by convection and PA1 (c) by way of the heated refractory lining of the rotary kiln.
The last-mentioned part of the heat is called stored heat and consists of that quantity of heat which is taken up by the refractory lining while it is in direct contact with the flue gas in the free space in the kiln and which is subsequently delivered by the refractory lining to the charge when the heated lining is being submerged under the charge which is traveling through the rotary kiln.
An essential problem arising in rotary kiln technology resides in that the various heat transfer modes, which are governed by different physical laws, must be matched to each other and to the process to be carried out as well as to the behavior of the charge materials. In this connection the fact that various materials which are to be thermally treated tend to form deposits and crusts on the inside surface of the kiln is of decisive significance. The formation of deposits and crusts is influenced by the chemical and physical properties of the various materials which are charged or of mixtures thereof or of any new materials formed by them and is closely related to the heat transfer and particularly to the temperature difference between the inside surface of the kiln and the charge material contacting said inside surface at a given point of the kiln. For instance, if the inside surface of the kiln is at an excessively high temperature, much more heat may of offered to the charge in contact with that inside surface than can be taken up by that charge, dissipated into the interior of the charge, and consumed for reactions in the time concerned. As a result, the charge particles directly contacting the inside surface of the kiln will be overheated and may superficially melt so that the particles may adhere to the inside surface of the kiln and to other charge particles. The effect of that phenomenon on individual particles will increase as their particle size and their volume related to their surface area decrease.
This crust-forming mechanism is typical for rotarty kiln processes, particularly for those mentioned above, and when occurring pronouncedly will result in trouble in operation so that it may be necessary to stop the rotary kiln for a removal of the crusts. A small deposit may be desirable for the protection of the refractory lining but should not grow in excess of the desired thickness.
Particularly during an interruption of the operation of the rotary kiln for a short time, for instance because there is a power failure and the rotary kiln is driven by emergency drive means or because the main drive means are operated at an extremely low speed, the heat transfer will be no longer uniform and the heat which is released will not be consumed at the usual rate. This will result in a retention of heat so that the refractory lining and the charge will be overheated and the overheating will result in a sintering of the charge and in a formation of crusts, particularly in the hotter zones of the rotary kiln.
It is known that a formation of crusts before a melting and sintering zone can be prevented in that the interior of the kiln is directly cooled at the charging end in the region in which crusts might form. Specifically, water can be sprayed into the interior and into the charge (German Utility Model No. 1,726,762). If the rotary kiln is cooled by a water jacket or by cooling pipes, the water which is sprayed is tapped from the cooling conduits in the kiln. That process requires an introduction of water into the rotary kiln and can be used only if the tendency to form crusts is restricted to one region. Otherwise, a large number of through bores and feed conduits would have to be distributed over the length of and around the periphery of the kiln.
In the production of cement it is also known to maintain the temperature of the refractory lining above the temperature at which a crust may form. The inside surface of the kiln is maintained at such a high temperature that any melting or sintering constituents cannot adhere to and solidify on the refractory lining (Austrian Patent Specification No. 231,339). But that process cannot be adopted to solve the problems involved in the formation of crusts in rotary kiln processes carried out below the melting and softening points of the charge components.
Numerous processes to protecting the refractory lining or the shell of the kiln have been disclosed. For instance, it is known to provide water-cooled cooling boxed around length sections of the kiln (German Patent Specification No. 2,495; German Patent Specification No. 116,732; Early German Disclosure No. 21 45 188) or to provide cooling pipes in the refractory lining in order to effect a cooling which in some cases causes a slag layer to be formed on the refractory lining (German Patent Specification No. 13,136; 21,220; 1,043,365; 1,006,780; 1,010,444) or to sprinkle water on the entire shell of the kiln or on part thereof so as to cause a slag layer to form (U.S. Patent Specification No. 939,817; German Patent Specification No. 698,732) or to provide stationary cooling pipes, which surround the shell of the kiln and in some cases permit of a utilization of the heat which is transferred by the cooling fluid (German Patent Publication No. 23 37 862; German Early Disclosure No. 27 47 457). But these processes do not solve the problem how to prevent the formation of undersired crusts.
It is an object of the invention to prevent in a rotary kiln formation of crusts during normal operation and when the kiln is operated at a reduced speed during short-time trouble.
This object is accomplished according to the invention in that several sections of the rotary kiln are cooled to effect a controlled dissipation of heat so that the temperature of the inside surface of the kiln in the portion which is being submerged under the charge is always maintained at or up to 50.degree. C. below the temperature of the charge disposed thereover. When the inside surface in a given cross section of the rotary kiln has risen above the rolling bed formed by the charge, the temperature of said inside surface rises as far as to that portion of the inside surface which is moving below the rolling bed. That portion is described as the portion which is being submerged. The inside surface is formed either by the refractory lining or by the desirable covering layer. The cooling is effected in such a manner that the temperature of that part of the inside surface which is not covered by the rolling bed but exposed does not rise above the temperature of the charge or does so rise and is lowered to a point not above the charge until it is being submerged. If the inside surface is maintained at the described temperature which is at or up to 50.degree. C. lower than the temperature of the charge adjacent thereto, formation of crusts is reliably avoided and an unnecessary dissipation of heat from the charge to the cooling fluid is also avoided. Cooling is effected in those length sections of the rotary kiln in which the temperature of the charge is already so high that a further temperature rise caused by a heat transfer from the hotter inside surface of the kiln results in a softening or melting of charge constituents and in formation of crust. For safety reasons, the cooling may begin a certain distance before that point so that temperature fluctuations are compensated.
According to a preferred feature, cooling is effected by sprinkling water in a controlled manner on sections of the of the shell of the kiln. In that manner, cooling is effected with a low expenditure.
According to a further preferred feature, water is sprinkled on the shell of the rotary kiln when the latter is operated at a reduced speed or is stopped during short-time troubles. Such troubles occur during a power failure, when the rotary kiln is driven at a lower speed by emergency drive means or during short-time repairs. The water rate is controlled so that the surplus heat is dissipated and there is no formation of crusts and sintering. The water can be kept available in an overhead tank if a power failure would result also in a failure of the water supply and there is no emergency supply. That cooling can be provided for rotary kilns which are not cooled during normal operation as well as for rotary kilns which are cooled during normal operation.
According to a further preferred feature, water is sprinkled on those sections of the shell in which a retention of heat causes the temperature in the charge to exceed the highest permissible temperature. The sprinkling of water begins shortly before the section in which the charge reaches that temperature so that an unnecessary cooling of the preceding sections is avoided.
According to a further preferred feature, the means for sprinkling water is connected to the interlock system and their operation is initiated automatically in response to a trouble in the operation of the kiln. In that case, a trouble in the operation of the kiln automatically initiates a cooling and overheating is safely avoided.
According to a further preferred feature, at least a major part of the water is sprinkled on that portion of the shell of the kiln which is opposite to the rolling bed formed by the charge. This avoids a lowering of the temperature of the inside surface of the kiln to an unnecessary low value also in that portion which is covered by the rolling bed and a dissipation of heat from the charge.
According to a further preferred feature, cooling is effected by means of pipes which are installed in the refractory lining of the kiln and flown through by a cooling fluid and the heat absorbed by the cooling fluid is utilized for other purposes. The cooling fluid consists usually of water and its heat content is utilized when the water has left the kiln. In that case, part of the dissipated heat can be utilized for useful purposes so that the structural expenditure is justified in some cases.
According to a further preferred feature, more energy is supplied to the rotary kiln than is required for carrying out the process and the surplus is utilized to heat the cooling fluid in the pipes to a temperature which is favorable for other purposes. In this way the temperature of the cooling fluid can be increased so that its heat content is available on a more valuable level. In that case heat can be dissipated also from sections which would not be cooled otherwise.
According to a further preferred feature, the temperature of the inside surface of the rotary kiln and the temperature of the charge are measured in several sections of the rotary kiln and the dissipation of heat is controlled in dependence on the temperature difference. That arrangement is particularly useful if the rotary kiln is operated under conditions which do not remain constant permanently or for prolonged periods of time. In the manner described, the rate at which heat is dissipated by cooling can be minimized while a formation of crusts is nevertheless reliably prevented.
According to a further preferred feature, a rotary kiln used to produce sponge iron is cooled through the length of the reducing zone and that cooling begins approximately where the wustite begins to form. In this way best results are produced with a low expenditure in that process.